Residential, commercial, agricultural, and industrial water usage generates a tremendous volume of wastewater that, left untreated, can present health hazards and environmental concerns. Accordingly, most municipalities employ wastewater treatment facilities downstream of their sewage collection systems to process raw wastewater into a more innocuous form.
Wastewater entering a treatment facility (influent) is about 99% water and 1% a mixture of large objects, solids, dissolved solids, oils, waxes, and organic matter. The latter portion can be separated from the former using a series of treatment steps often termed preliminary treatment, primary treatment, secondary treatment, and advanced treatment. Preliminary and primary treatment removes of solids. Secondary treatment removes dissolved organic matter using microorganisms. Advanced (tertiary) treatment removes nutrients such as nitrogen and phosphorous. Other treatments include disinfection, filtration, and reverse osmosis.
The end products of conventional wastewater treatment systems are treated wastewater and sludge. The treated wastewater is typically disposed of in a body of water (e.g., a stream, river, or ocean) adjacent to the wastewater treatment plant. Sludge is typically dumped at an offsite location, but is sometimes processed for use as fertilizer.
The creation and disposal of these end products as well as the general operation of a wastewater treatment plant can have undesirable effects. For example, the cumulative discharge of treated wastewater into an adjacent body of water can be deleterious to the environment. In addition, the production of sludge can create strong odor which renders the land surrounding the treatment plant undesirable for habitation or occupation.